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10.
Other miscellaneous Organic Redox
Reactions
This is a
'collection' of reactions not dealt with in sections 8. and 9. They
may/may not be useful reactions.
Section 10.
reaction sub-index: 10.1 Cannizzaro reaction * 10.2
aldehydes/ketones tests * 10.3
Combustion * 10.4 Fuel cells
-
10.1 The Cannizzaro reaction
-
Aldehydes
which do not have a hydrogen atom on the carbon next to the carbon
of the carbonyl group (C=O) undergo the Cannizzaro reaction with
concentrated aqueous sodium hydroxide in which one molecule
of the aldehyde is reduced to a primary alcohol and another is
oxidised to the sodium salt of a carboxylic acid.
-
This is
an organic example of disproportionation in which the same
carbon atoms of the reactant molecule simultaneously increase and
decrease their oxidation state e.g.
-
10.2 Simple chemical tests to distinguish aldehydes from ketones
-
The tests
depend on the relative redox stability of ketones compared to the
much more readily oxidised aldehydes. These tests also give
positive results with many reducing sugars and some rather more
stable aromatic aldehydes e.g. benzaldehyde, may not give a positive
result at all.
-
Tollens
reagent is a colourless solution of silver nitrate in aqueous
ammonia.
-
When an aldehyde is warmed with
Tollens reagent it is oxidised to a
carboxylic acid and the silver ion (in ammine complex form) is
reduced to silver, forming a silver mirror on the side of the test tube.
-
2[Ag(NH3)2]+(aq)
+ R-CHO(aq) + H2O(l) ==>
2Ag(s) + 4NH3(aq) + R-COOH(aq)
+ 2H+(aq)
-
simplified: 2Ag+(aq)
+ R-CHO(aq) + H2O(l) ==>
2Ag(s) + R-COOH(aq)
+ 2H+(aq)
-
or 2[Ag(NH3)2]+(aq)
+ R-CHO(aq) + 2OH-(aq) ==>
2Ag(s) + 4NH3(aq) + R-COOH(aq) +
H2O(l)
-
simplified: 2Ag+(aq)
+ R-CHO(aq) + 2OH-(aq) ==>
2Ag(s) + R-COOH(aq) +
H2O(l)
-
Ketones show no reaction because of their
greater stability to oxidation.
-
Fehlings or
Benedict's solution consists of a copper(II) ion complexed with
an organic carboxylic acid.
-
The deep blue copper(II) ion in the
complex is reduced to a red-brown precipitate of copper(I) oxide.
-
Ketones show no reaction
due to their greater oxidation stability.
-
2Cu2+(complex/aq)
+ R-CHO(aq) + 2H2O(l)
==> Cu2O(s) + R-COOH(aq) + 4H+(aq)
-
or: 2Cu2+(complex/aq)
+ R-CHO(aq) + 4OH-(aq)
==> Cu2O(s) + R-COOH(aq) + 2H2O(l)
-
Ketones show no reaction because of their
greater reluctance to oxidation.
-
There are lots
more organic chemical tests described -
Chemical Identification Tests (with alphabetical index).
-
10.3
All
organic compound air/oxygen combustion reactions are oxidations in terms of
the carbon atoms of the organic molecule e.g. carbon's oxidation state
is increased from (-4) in methane to (+4) in CO2 for complete combustion and
(+2) if carbon monoxide formed or (0) in carbon (soot) if the
combustion is
inefficient/incomplete. In each case oxygen's oxidation state changes
from (0) to (-2) to offset the increase in carbon's oxidation state. e.g.
-
CH4(g) +
2O2(g) ==> CO2(g) + 2H2O(l)
-
2CH4(g) + 3O2(g) ==>
2CO(g) + 4H2O(l)
-
CH4(g) + O2(g) ==>
C(s) + 2H2O(l)
-
 10.4
Organic Fuel Cells
-
Many hydrocarbon
molecules are burned as fuels, but since it is a redox reaction,
theoretically it can be done as a combined half-cell oxidation/reduction electron transfer reaction.
-
In practice organic compounds can be oxidised by oxygen
in a way that can be used to generate electricity directly in a fuel cell
(right diagram) rather than release the energy as heat e.g.
-
In reaction 9.1(a)
ethanol was oxidised to ethanoic acid by acidified potassium
dichromate(VI). The same result can be
obtained by reaction of ethanol with oxygen in a fuel cell.
-
The reaction is very exothermic,
so ethanol is a good source of chemical potential energy.
-
Ethanol is used directly as a fuel in a direct ethanol fuel cell (a
DEFC fuel cell).
-
(i) CH3CH2OH(aq)
+ O2(g) ==> CH3COOH(aq) + H2O(l)
(ΔHØ = -494 kJ mol-1)
-
The half-cell
reactions are:
-
(ii) Reduction,
+ve electrode: O2(g) + 4H+(aq)
+ 4e- ==> 2H2O(l) (EØ
= +1.23V)
-
(iii) Ox'n,
-ve electrode: CH3CH2OH(l)
+ H2O(l) ==> CH3COOH(aq)
+ 4H+(aq) + 4e- (EØ
= +0.06V)
-
Adding (ii) +
(iii) = equation (i) and EØcell = EØ+/red'n
- EØ-/ox'n = 1.23 - 0.06 = +1.17V
-
The reactions must
take place on catalytic electrodes made of platinum and other
transition metals and here the inner 'electrolyte' is as a polymer
proton exchange membrane of the fuel cell (a PEFC fuel cell), though it can be a
concentrated phosphoric acid solution (in a PAFC fuel cell).
-
The electrons will
flow through the external circuit from the -ve electrode to the
+ve electrode.
-
Free energy
change: ΔGØ = -nEØF = - 4 x 1.17 x 96500
= -451620 J mol-1 = -451.6 kJ mol-1
-
n = number
of electrons transferred, EØ = cell voltage,
F
= Faraday constant in coulombs mol-1
-
This sort of
chemistry is being developed to make reasonably efficient portable
fuel cells
-
Commercially developed fuel cells will
hopefully convert the ethanol completely into water and
carbon dioxide to give a greater and more efficient energy output, but
its still generates a
voltage of 1.0-1.2V per cell, in this case ...
-
(i) Oxidation,
-ve electrode: C2H5OH(l) +
3H2O(l) ==> 12H+(aq)
+ 2CO2(aq/g) + 12e-
-
(iii) Reduction,
+ve electrode: 3O2(aq/g) + 12H+(aq)
+ 12e- ==> 6H2O(l)
-
adding (i) + (iii)
gives: C2H5OH(l) + 3O2(aq/g)
==> 3H2O(l) + 2CO2(aq/g)
-
+ other organic
products as the reaction is not completely efficient.
-
The cells can
obviously be connected in series to give larger voltages.
-
The ethanol
('alcohol', C2H5OH) can be bio-sourced from
sugar beet, potatoes and cereal crops and other plant material that
can fermented with enzymes. The ethanol is fractionally distilled from
the fermented mixture and constitutes a renewable fuel.
-
However there seems to
have been more work done? on the Direct Methanol Fuel Cell (a DMFC
fuel cell),
though there are concerns over methanol's toxicity and the very costly
platinum catalytic electrodes required, but the DMFC is essentially like the DEFC described above and
the principles illustrated in the
diagram.
-
The methanol can be synthesised in the
reforming reaction CO(g) + 2H2(g)
==> CH3OH(l)
-
Hopefully,
efficient cells using ethanol can be developed because ethanol
compared to methanol has a higher energy density (e.g. kJ/kg), is less
toxic and can be bio-resourced as a renewable fuel.
-
There is a huge
amount of research going on in fuel cell development to try to use cheaper,
but equally effective tiny particle metal catalysts of Fe, Co and Ni
instead of costly platinum, but the most efficient metals are still
the most costly?
11.
Oxidation state and organic compounds
Usually the oxidation state
of hydrogen is +1, and oxygen -2 in organic compounds.
On this basis you can
achieve a useful oxidation number analysis of simple organic compounds
in an oxidation sequence.
e.g. the oxidation
sequence below, with the oxidation state of
carbon in () and in hydrogen
in ().
CH4 (-4)
== ox'n ==> CH3OH (-2) ==
ox'n ==> HCHO (0)
== ox'n ==> HCOOH (+2) ==> CO2
(+4)
CH3CH3 (-3,-3)
= ox'n => CH3CH2OH (-3,-1) =
ox'n => CH3CHO (-3,+1)
= ox'n => CH3COOH (-3,+3)
Note the rise of
carbon's oxidation state in increments of 2, see oxidation equations for
acidified potassium dichromate(VI) reaction with alcohols and aldehydes
in section 9.1(a) where the half-cell
oxidation equations involve a 2 electron loss from the organic molecule. Other organic molecules
and redox sequences can be similarly 'analysed'
ethene H2C=CH2
(-2,-2) + H2
(0)
== reduction/Ni
==> ethane CH3-CH3 (-3,-3), (+1)
propene CH3-CH=CH2
(-3,-1,-2) + H2
== reduction/Ni ==> CH3-CH2-CH3
(-3,-2,-3)
ethanol CH3-CH2-OH
(-3,-1)
== ox'n
==> ethanal CH3CHO (-3,+1)
== ox'n ==> CH3COOH (-3,+3)
GENERAL
REVISION
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N
==> RCH2NH2
In the
laboratory, reacting a nitro-aromatic with a mixture of tin and conc.
hydrochloric acid by 
The 25% sulphuric
acid is placed in the flask and gently simmered. The alcohol and aqueous
sodium/potassium dichromate(VI) solution is dripped onto the hot acid.
Immediately, the orange dichromate(VI) is reduced by the alcohol to the
green chromium(III) ion and the alcohol is oxides to the aldehyde or ketone.
Hydrocarbons are
difficult to oxidise with typical organic oxidising agents compared to
compounds like alcohols. However, aromatic hydrocarbons with an alkyl side
chain can be oxidised with strong reagents such as aqueous potassium
manganate(VII)/sodium hydroxide.
+ 6[O] ==>
+ 2H2O
